Field of the Invention
The invention relates to a variant of a threaded connection including an outer part with an internal thread and an associated inner part with an external thread. The respective threads of the parts have a uniform lead. The individual thread has a substantially V-shaped profile and at least one of the threads is furnished with a wedge ramp at the thread root wherein when the inner and outer parts are screwed into one another the thread crests of the one threaded connection part abut with the wedge ramps at the root of the thread of the other connection part.
The invention further relates to another variant or special configuration of a threaded connection including an outer part with an internal thread and an associated inner part with an external thread. The respective threads of the outer and inner parts have a uniform lead. The individual thread of the one part has a substantially V-shaped profile and the other part is furnished with an encircling wedge ramp in the nature of a thread. When the inner and outer parts are screwed into one another the thread crests of the one threaded connection part abut with the encircling wedge ramp in the nature of a thread of the other connection part.
The invention further relates to the use of both variants of such threaded connections as connections for carbon, semigraphite or graphite electrodes that are locking, load-bearing, and not susceptible to unscrewing, especially under dynamic loading.
The technique of manufacturing carbonized or graphitized carbon, also including carbon electrodes and connecting pins therefor, has been known in the art for over a hundred years and it is applied on a large industrial scale. Accordingly, it has been refined in many respects and optimized in terms of costs. One description of this technology may be found in ULLMANN'S ENCYCLOPEDIA OF INDUSTRIAL CHEMISTRY, Vol. A5, VCH Verlagsgesellschaft mbH, Weinheim, 1986, pp. 103-113.
An arc furnace contains at least one column of carbon electrodes. The upper end of such a column is retained by a bracket, through which the electrical current for the electrode column is also supplied. When the furnace is in use, the electric arc passes from the bottom tip or lower end of the column into the metal for melting that is located in the furnace. The electric arc and the high temperatures in the furnace cause the bottom end of the electrode column to burn away slowly. The shortening of the electrode column is compensated in that the column is advanced progressively into the furnace, and if necessary a further electrode is screwed onto the top end of the column. If necessary, a partially burned off column consisting of several electrodes and their connecting pins may also be removed from the bracket as a single unit and replaced by a fresh column of sufficient length.
Individual carbon electrodes are screwed onto a column already situated in the furnace, or electrodes are screwed to a fresh column either by hand or with a machine. Particularly in the case of electrodes having a large diameter of 600 mm or more, significant forces and turning moments or screwing effort must be applied in order to ensure that an electrode column will not come apart. Secure attachment of a column is vitally important for the functioning of an arc furnace.
The secure attachment of a column is threatened during transport, but particularly when a furnace is in operation. When a furnace is in use, considerable flexing moments are exerted repeatedly on the electrode column due to the oscillation of the furnace casing including the column, or the column is subjected to constant vibration; the column is exposed to impacts from the charge material, which also places stresses on the secure attachment of the column. All such stresses—repeated flexing moments, vibrations and impacts—are capable of causing the threaded connection of electrodes to loosen. Loosening must be considered to be the result of unavoidable and/or undesirable processes.
For the sake of better understanding, the consequences of an electrode column coming loose while the furnace is in operation will be described:                Loosening of the column is an indication that the tightness of the screw attachment is reduced. As a result, the compressive forces on the contact surfaces of adjacent column elements are also lessened. Loosening may progress until some contact surfaces become physically separated from each other.        As a result, the electrical resistance in the connection is increased. Those surfaces that are still in contact are subjected to greater current density. The higher current density leads to localized thermal overheating.        When a screw connection becomes loose, the connecting pin is usually exposed to a high thermal and mechanical load. Ultimately, mechanical failure of the connecting pin due to overheating and mechanical loading is to be expected. As a result, the lower end of the electrode column breaks off and falls into the molten steel, the electric arc is interrupted and the smelting process is terminated.        
In order to counter the problems of inadequate attachment and poor current transfer from one part of the electrode column to the next, a number of very different approaches have been instituted. The practice described in the following is also implemented in the steelworks.
U.S. Pat. No. 4,167,643 described that the connecting pin between two graphite electrodes had a lower coefficient of thermal expansion than the two electrodes. As temperatures rose—in steelworks operations, temperatures well above 1500° C. are reached—the electrode sections expanded more than the connecting pin. This caused additional, heat-induced tightening between the connecting pin and the electrodes, which were considered to be a safety cut-out for the threaded connection. However, it was also evident that the thermally induced forces placed the flanks of the thread windings under severe stress.
U.S. Pat. No. 5,575,582 described the use of a tapered cutout pin in addition to the connecting pin between two graphite electrodes. With the electrode column in the screwed state, the arrangement ensured that the cutout pin in a recess extending about half through the contact surfaces of two adjacent electrodes fell out of the recess in the respective upper electrode of the column and into the recess in the corresponding lower electrode in the column. The recess in the lower electrode was conformed as a curved channel that traced an arc having constant radius about the central longitudinal axis of the electrode. When the threaded connection became loose, the cutout pin was able to slide along the channel until it was blocked at the farther end. The cutout pin that had fallen down into the recesses thus prevented further distortion and consequent loosening of the two electrode sections. Depending on the length of the curved channel and the torsional path about the central longitudinal axis of the electrode that the cutout pin traversed therein, the threaded connection was still capable of becoming quite loose. This had deleterious effects on the transfer of current through this threaded connection and in terms of localized overheating at this connection.
In other fields too, attempts have been made to resolve the problem of fixtures becoming loose. In German published patent application DE 41 37 020, self-protecting fixtures were described such as screws and nuts made from materials not further described. The fixture was furnished with a number of knob-like projections in the frontal surface that cooperated with a structural member. The projections were conformed as pyramids or cones having a height of less than 1 mm, wherein the angle at the tip of the pyramid or cone was at least 90°. The pyramids or cones were intended to be impressed into the surfaces of the structural members to be braced upon screwing tight, thus preventing the fixtures from coming unscrewed. Reference was had to “setting” and the associated reduction in prestressing (see col. 2, line 9, of the German application). The pyramids or cones were distributed evenly over the frontal surface of the fixture. The fixture had no specifically structured contact surface, and accordingly no front tension direction with special effectiveness.
With reference to screwing columns of carbon electrodes, it should be noted that macroscopic knobs on the contact surfaces of the electrodes or connecting pins would be smashed during screwing because of the ceramic and thus brittle nature of the carbons. It is even possible that substantial pieces might be ejected from the frontal surfaces of the column elements.
A different approach to preventing fixtures from becoming loose was described in the U.S. Pat. No. 4,076,064 (1978). A wedge ramp was introduced at the root of the thread winding of the one component of a threaded connection. When both components of the threaded connection were screwed together, the crests of the thread windings of the threaded connection component without the wedge ramp abutted with the wedge ramp at the root of the thread winding of the other component. The abutment of the thread winding crests of the one threaded connection component with the wedge ramp at the root of the thread winding of the other component had the effect of locking both components. This locking effect was improved if both components were made from appropriately selected materials. It was helpful if the bolt in the threaded connection was made from a harder, less ductile material than the associated nut. Since the materials were not described in greater detail, the logical assumption was that for the purposes of this patent, metallic materials were concerned. No indication was given that this type of threaded connection was also created using materials made from synthetically produced carbons, nor for tapered threaded connections as are commonly used to connect carbon and/or graphite electrodes as described herein.
The element having the wedge ramp at the root of the thread winding was also dealt with in U.S. Pat. No. 4,266,590 (1981). In that case, heights of the thread windings in the nut and the bolt were slightly different. As a result, for each thread winding the thread winding crests of the threaded connection component without the wedge ramp were located at different relative positions in the clear cross-section of the thread winding with the wedge ramp on the thread winding root of the other component. According to this patent specification, the locking effect that was achieved by the thread crests striking the wedge ramp was further reinforced by a jamming effect due to the differing heights of the thread windings in the two threaded connection parts. No indication was given that this type of threaded connection was also created using materials made from synthetically produced carbons, nor for tapered threaded connections as are commonly used to connect carbon and/or graphite electrodes as described herein.
In the practical running of a steelworks, one attempts to screw the electrodes together as firmly as possible. As was indicated in the preceding, the forces, turning moments and screwing effort that can be applied manually are limited. These forces may be increased considerably using machinery, but such mechanical screwing devices are only utilized in a few steelworks. Actual steelworks operations indicate that elements of the electrode columns still come loose repeatedly.